1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly to a gain-clamped semiconductor optical amplifier having photo detectors which are integrated on a single crystal substrate and can detect optical intensities at input/output terminals of the optical amplifier, and a method of fabricating the gain-clamped semiconductor optical amplifier.
2. Description of the Related Art
In a general optical communication system, light emitted from a transmitter that is transmitted through an optical transmission line suffers transmission loss that reduces the signal arriving at a receiver. When the power of light arriving at a receiver is smaller than a predetermined value, the receiving error prevents normal optical communication from being performed. Therefore, an optical amplifier is provided between a transmitter and a receiver so as to amplify light, thereby compensating for the transmission loss of the light transmitted through the optical transmission line and enabling the light to be transmitted to a farther distance with little error.
Such optical amplifiers include an erbium-doped fiber amplifier (hereinafter, referred to as EDFA), a Raman amplifier, and a semiconductor optical amplifier (hereinafter, referred to as SOA).
The EDFA, which uses an optical fiber doped with the rare-earth elements, e.g., erbium, for amplification, has a high gain characteristic, a low noise figure (NF), and high saturation output power. EDFA has accordingly been widely used in a backbone network or in a metro network. However, the EDFA is expensive and affords an operation wavelength that is limited to a 1.5 μm band.
The Raman amplifier uses a Raman amplification in an optical fiber. Raman amplification is a method for amplifying light using a so-called Raman amplification phenomenon. According to Raman amplification, when a pumping light of a strong light is incident into the optical fiber, a gain appears at a longer wavelength side distanced about 100 nanometers (nm) from the wavelength of the pumping light due to stimulated Raman scattering. Light of the wavelength band having the above-described gain is incident into the excited optical fiber, so that light is amplified. The Raman amplifier can easily adjust an amplification band by properly setting the wavelength of the pumping light for the Raman amplification, and has a low noise figure. However, the Raman amplifier not only has very low optical amplification efficiency but also needs a high-priced pumping light source, thereby increasing the entire size of the optical amplifier module and the price of the optical amplifier module.
The SOA uses gain characteristics of a semiconductor and can adjust its amplification band according to a semiconductor band gap. The SOA advantageously features a small size of a few centimeters (cm) and, notably, does not require a high-priced pumping light source.
However, the SOA generally suffers a gain saturation phenomenon in which the gain value decreases with increase in the intensity of the input signal. Amplification for transit of a signal having large optical power therefore causes signal distortion.
In order to solve this problem, a gain-clamped SOA having a structure as shown in FIG. 1 has been proposed.
FIG. 1 is a view showing a structure of a conventional gain-clamped semiconductor optical amplifier (gain-clamped SOA) 100. The gain-clamped SOA 100 includes an n-InP substrate 101, an InGaAsP passive waveguide layer 102, an InP spacer 103, a DBR lattice pattern 104, an active-layer waveguide 105, a current blocking layer 106, a p-InP clad layer 107, a p-InGaAs layer 108 for reducing an ohmic contact resistance, an oxide layer 109, an upper electrode 110, and a lower electrode 111.
The gain-clamped SOA 100 induces laser oscillation in a short wavelength, far from a wavelength range of an input signal to be amplified, by using both distributed Bragg reflector lattices, thereby fixing the density of carriers in a resonator, so that optical gain is constantly maintained even though a driving current increases.
However, in the conventional gain-clamped SOA, a first procession direction (shown as “A” in FIG. 1) of a signal is the same as a second procession direction (shown as “B” in FIG. 1) of a laser beam to induce oscillation. Therefore, when signals of several channels are amplified, a four wave mixing phenomenon is caused between an oscillation wavelength and a signal wavelength. Further, the conventional gain-clamped SOA has another problem in that a separate wavelength filter is required for removing the oscillation wavelength of the laser.
Meanwhile, in order to control a gain of the gain-clamped SOA or check whether or not a device is properly operated, it is necessary to know intensities of an inputted signal and an amplified/outputted signal. To this end, conventionally, after a portion of optical power of the signal inputted to the amplifier and a portion of optical power the signal outputted from the amplifier are separated by means of optical dividers, each portion is inputted to a photo detector to be measured.
However, according to the conventional art, since a portion of a signal is separated, optical power is lost, thus degrading important properties of the optical amplifier, such as noise, saturation optical output, gain, etc. Also, since at least one optical divider and at least one photo detector are additionally required to detect a portion of an optical signal, the number of components and processes increases, making a competitive price for the product more difficult to realize.